A high intensity discharge lamp (HID lamp) is used for a light source apparatus for an image display optical device such as an LCD projector and a DLP (Trademark) projector. In such a projector, light is separated into the three primary colors of red, green, and blue by a dichroic prism, etc., so that a space modulation element provided for each color generates an image of each of the three primary colors. The optical paths thereof are combined by a dichroic prism, etc., so as to display a color image. In another known type of projector, light emitted from a light source is passed through a rotating filter having three primary color areas (R, G, and B), thereby sequentially generating light rays of the three primary colors. In synchronization with the generated light rays, the spatial modulation device is controlled so as to sequentially generate an image of each of the three primary colors in a time dividing manner, thereby displaying a color image.
As to types of driving methods of a discharge lamp in a steady lighting period, there are a direct-current driving method and an alternating current driving method in which periodic polarity reversals are performed by further providing an inverter. In the case of the direct current driving method, since the light flux from a lamp is like direct current, that is, it does not change with passage of time, basically, there is a big advantage that it can be similarly applied to both types of the above-described projectors. On the other hand, in the case of the alternating current driving method, while development or wear of the electrode(s) of the discharge lamp can be controlled by using the flexibility of polarity-reversal frequency which does not exist in the direct-current driving method.
When this type of a lamp is started, while voltage called no-load open circuit voltage is applied to the lamp, high voltage is applied to the lamp, in order to generate dielectric breakdown in an electrical discharge space, so that the discharge state changes from glow discharge to arc discharge. As a conventional method of carrying out the start-up in a case of an alternating current driving method, there has been a resonance starting which is accomplished by a series resonance system, in which a series resonant circuit made up of a resonance inductor and a resonant capacitor is provided in an output side of an inverter, wherein at a time of start-up, polarity frequency of the inverter is set up so as to agree with the resonance frequency of the resonant circuit, thereby generating a series resonance phenomenon, so that voltage to be applied to the lamp is increased. Furthermore, by using the resonance starting in combination with an igniter, a peak value of high voltage to be applied thereto is increased thereby increasing starting probability.
FIG. 18 is a schematic view of the structure of an example of a conventional discharge lamp lighting apparatus. The principle of resonance starting will be described below, referring to FIG. 18. The discharge lamp lighting apparatus shown in the figure comprises a power supply circuit (Ux′) which supplies electric power to a discharge lamp (Ld), a full bridge type inverter (Ui′) for inverting the polarity of an output voltage, and a resonance inductor (Lh′) and a resonant capacitor (Ch′), wherein at a start-up time, the inverter (Ui′) is driven in an polarity-reversal driving operation at resonance frequency determined by a value of the product of the inductance of the resonance inductor (Lh′) and the electrostatic capacity of the resonant capacitor (Ch′), or at frequency close to the resonance frequency, so that high voltage is generated between both terminals of the resonant capacitor (Ch′) due to an LC series resonance phenomenon which is developed by the driving, whereby the high voltage is applied to the discharge lamp (Ld).
Although, in order that resonance current which flows through the inverter (Ui′) does not become excessive, it is necessary to make the electrostatic capacity of the resonant capacitor small. And to increase the inductance of the resonance inductor to some extent at time of the series resonance operation, if the inductance thereof is large, it tends to cause instantaneous interruption of lamp flux, overshoot, and vibration at a time of steady lighting. And, in such a series resonance system, in order to sufficiently raise voltage applied to a lamp to start up, frequency of a periodic voltage applying unit or frequency of a higher harmonic component thereof needs to be set up so as to agree (namely, be synchronized) with the resonance frequency or oddth frequency of the resonance frequency of the resonant circuit.
However, since there is manufacturing tolerance in parts, even if the inverter (Ui′) is driven in a polarity-reversal driving operation at a predetermined and fixed frequency determined by the design inductance of the resonance inductor (Lh′) and the design electrostatic capacity of the resonant capacitor (Ch′), there is a problem in which expected high voltage cannot be obtained. Furthermore, in such a case where there is the manufacturing tolerance, although the resonance frequency of each discharge lamp lighting apparatus may be measured so as to set it up, since there are also effects of the length of cables for connection and a degree of a proximity of the cables to other electrical conductor, etc., there is a problem in which it is difficult to rigorously set up the resonance frequency in advance.
In order to solve this problem, a method of setting up the driving frequency of the inverter (Ui′) to the above-mentioned resonance frequency or frequency close to the driving frequency or a method of performing a sweep operation, is proposed in the prior art. FIG. 19 is a simplified timing chart of an example of a conventional discharge lamp lighting apparatus. In the figure, (a) shows a waveform of output voltage (Vnh) which is generated in the resonant capacitor (Ch′), and (b) shows change of the driving frequency (f) of the inverter (Ui). This figure shows that an automatic sweep operation of frequency of alternating current voltage, which the inverter (Ui′) generates at a time of start-up of lamp lighting, is repeatedly performed in a predetermined range including the resonance frequency of the resonant circuit, wherein, in a period (Ta), the sweep operation is performed from a lower limit frequency towards a upper limit frequency, and in a process of the operation, the output voltage (Vnh) turns into high voltage at a time point (ta) at which the frequency of the alternating current voltage generated by the inverter circuit is in agreement with the resonance frequency by chance. On the other hand, in a period (Tb), the sweep operation is performed in an opposite direction thereto from the upper limit frequency towards the lower limit frequency. Therefore, the sweep operation is repeated twice or more, in a range of resonance frequencies expected from the manufacturing tolerance, within a predetermined period (T) of start-up time of the lamp lighting, and the high voltage is applied to the discharge lamp (Ld). The peak voltage of this high voltage is set up so as to be 2 kV-5 kV (since the peak voltage is obtained by measuring voltage reaching the peak value from 0 V. The way of measuring the peak of high voltage of alternating current is the same throughout the present specification).
In a period, in which the driving frequency of the inverter (Ui′) is largely different from the resonance frequency or frequency close to the resonance frequency (in the figure, all the periods in which output voltage (Vnh) is relatively low, which are typified by a period (Tc)), and which is within the period (T), where high voltage is applied to a discharge lamp so as to start an operation, there is a problem in which, as to the output voltage (Vnh), a rise in voltage due to resonance does not occur at all.
Various proposals in technology such as one described above, have been made conventionally, in which a sweep operation of the driving frequency is repeated and continues over an entire discharge lamp start-up period, without specifying timing at which the driving frequency of an alternating current driving circuit is in agreement with the resonance frequency.
Japanese Patent Application Publication No. H02-215091 discloses that conditions, in which the driving frequency is in agreement with resonance frequency is satisfied at least for a moment, and an automatic sweep operation of the frequency of the alternating current voltage, which an inverter circuit generates at start-up time of lighting, is performed in a predetermined range including the resonance frequency of a resonant circuit.
Moreover, Japanese Patent Application Publication No. H03-102798 discloses that a high frequency unit, which makes an LC circuit apply a high voltage to a lamp, is provided so that the lamp may be lighted, wherein the high frequency unit applies, to the LC circuit, frequency that changes with passage of time or frequency that decreases from frequency higher than the resonance frequency with passage of time.
Moreover, Japanese Patent Application Publication No. H04-017296 discloses that when oscillation frequency of an inverter unit is changed into high frequency, it is configured so that the oscillation frequency may be changed within a predetermined range according to output voltage of a saw-tooth wave generating unit or a triangular wave generating unit.
Furthermore, Japanese Patent Application Publication No. H04-272695 discloses that an inverter is controlled so that output frequency of the inverter is continuously changed to frequency lower than a frequency range in which an acoustic resonance phenomenon may occur due to the resonance frequency of an LC circuit at start-up, or an inverter is controlled so that frequency thereof may become lower than a frequency range in which an acoustic resonance phenomenon may occur at stationary time.
Furthermore, Japanese Patent Application Publication No. H10-284265 discloses that frequency of alternating current voltage outputted from an output connection section in a start-up period is swept (changed) within a range including the resonance frequency of a resonant circuit, or alternating current voltage of high frequency is outputted from the output connection section in a start-up period and only the alternating current operating voltage of low frequency is supplied to a discharge lamp in a steady lighting period after the start-up of the discharge lamp.
Furthermore, Japanese Patent Application Publication No. 2000-195692 discloses that, as an embodiment, operating frequency of a bridge in a resonance operation is swept (varied) so as to pass a resonance point.
Furthermore, Japanese Patent Application Publication No. 2001-338789 discloses that the switching frequency of each switching element is controlled so as to be continuously changed for predetermined time, wherein the sweep range of the switching frequency includes resonance frequency determined by an inductor and a capacitor of a load resonance circuit, or frequency is controlled so as to be changed, that is, swept, from higher frequency to lower frequency during a predetermined period, or when the resonance frequency changes after insulation breakdown of a discharge lamp, frequency of an inverter is also changed, so that large energy is supplied to arc discharge, whereby a discharge state of the discharge lamp more stably shifts to arc discharge.
Furthermore, Japanese Patent Application Publication No. 2002-151286 discloses that, as an embodiment, a sweep operation of the driving frequency of an inverter is repeated twice or more times, and the frequency is changed and shifted from high frequency to low frequency in arc lighting.
Furthermore, Japanese Patent Application Publication No. 2004-146300 discloses that as an embodiment, although two resonance systems are used, a sweep operation is performed using a microprocessor, wherein the lower limit frequency and the upper limit frequency of a frequency sweep range are set so as to define a frequency variable range which can be covered even if the resonance frequency changes due to manufacturing tolerance of parts of a resonant circuit section or floating capacitance of an output line from a high pressure discharge lamp lighting apparatus to the lamp.
Furthermore, Japanese Patent Application Publication No. 2004-221031 discloses a discharge lamp lighting apparatus having a control unit for at least setting up frequency in a first step so as to be frequency close to that obtained by dividing resonance frequency of a resonant circuit by an odd number while gradually decreasing frequency of the rectangular wave, wherein the frequency and the duty ratio of a DC-DC converter circuit, which is arranged in an upstream side of an inverter, are changed in order to suppress resonance voltage due to manufacturing tolerance of LC parts.
Furthermore, Japanese Patent Application Publication No. 2005-038813 discloses that, as an embodiment, frequency of an inverter in a high frequency switching operation at start-up time is changed continuously or in a stepwise fashion, in order to perform oddth resonance.
Furthermore, Japanese Patent Application Publication No. 2005-050661 discloses that, as an embodiment, output frequency of an inverter is continuously changed from an upper limit to a lower limit in a discharge lamp start-up time, and if it reaches the lower limit, the same operation is repeated after returning to the upper limit, so as to pass a resonance point.
Furthermore, Japanese Patent Application Publication No. 2005-038814 discloses that, as an embodiment, although a half bridge function and a step down chopper function are attained by two switching elements, frequency of an inverter is swept by dividing it twice or more times, in order to perform and operate at start-up at frequency which is one divided by an odd number of the resonance frequency.
Furthermore, Japanese Patent Application Publication No. 2008-243629 discloses that, in order to obtain resonance frequency, a sweep operation of frequency of an inverter is repeatedly carried out, or frequency of an inverter in an unloaded condition, starting improving mode, and each mode in a steady lighting state, is set as follows: non-load condition>steady lighting state>starting improving mode.
Thus, the proposal of the prior art is described above, that is, a sweep operation of the driving frequency is repeated and continues over an entire discharge lamp start-up period, without specifying timing at which the driving frequency of an alternating current driving circuit such as an inverter is in agreement with the resonance frequency. However, as described above, in an operation period, in which the inverter (Ui′) is operated at a frequency largely different from the resonance frequency or from a frequency close to the resonance frequency, and which is within the period (T) where high voltage is applied to a discharge lamp so as to start an operation, the problem in which a rise in voltage due to resonance does not occur at all has not been solved.
In order to solve this problem, in the prior art, it has been proposed that driving frequency of the inverter (Ui′) is automatically synchronized with or set to the resonance frequency of the resonant circuit which is made up of the resonance inductor (Lh′) and the resonant capacitor (Ch′), or a frequency close thereto or a higher order resonance frequency.
Description of a discharge lamp lighting apparatus shown in FIG. 18 will be given below. The circuit includes an a full bridge type inverter (Ui′) for inverting the polarity of an output voltage, and a resonance inductor (Lh′) and a resonant capacitor (Ch′), wherein a polarity-reversal driving operation is performed at a resonance frequency or a frequency close to the resonance frequency, so that high voltage is generated between both terminals of the resonant capacitor (Ch′) due to an LC series resonance phenomenon which is developed by the driving, whereby the high voltage is applied to the discharge lamp (Ld). However, a synchronization degree detection unit (Un′), which serves as a detection unit for detecting whether resonant condition is realized, is provided in order to control the output voltage (Vnh).
FIG. 20 is a schematic timing chart of an example of a conventional discharge lamp lighting apparatus, in a case where the synchronization degree detection unit (Un′) for controlling the inverter (Ui′) relating to series resonance, is used. In the figure, (a) shows a waveform of output voltage (Vnh) generated in the resonant capacitor (Ch′), and (b) shows change of the driving frequency (f) of the inverter (Ui′). The figure shows frequency of alternating current voltage, which the inverter (Ui′) generates at a time of lighting start-up, is automatically changed in a range including a resonance frequency of a resonant circuit, wherein in a period (Td), a sweep operation is performed from a lower limit frequency towards an upper limit frequency, and at time (td), resonance is realized and the synchronization degree detection unit (Un′) formed from a voltage detection unit detects that output voltage (Vnh) reached a target voltage, so that the frequency (fp) is maintained thereby generating intended high voltage continuously.
Since the output voltage (Vnh) is set up so that peak voltage may be set to 2 kV-5 kV as described above, the synchronization degree detection unit (Un′) needs to have the capability of withstanding the high voltage. As an example of the detection unit for realizing the resonant condition, it is necessary to measure voltage between a connection node of a resonant capacitor (Ch′) and a resonance inductor (Lh′), and a ground, or between both ends of a discharge lamp (Ld), thereby generating a signal. For example, resistor elements and capacitors are in series aligned, in order to withstand the high voltage, so that a signal can be acquired from a middle point thereof at which voltage is divided. However, in such an example, since the number of component parts increases, there is a problem in which it becomes disadvantageous in view of a miniaturization and cost reduction of such a discharge lamp lighting apparatus.
Moreover, as another example of the detection unit for realizing the resonant condition, a secondary winding which has a small turn ratio suitable for a resonance inductor (Lh′) is added thereto, and a resonance inductor (Lh′) is configured so as to have a transformer structure, wherein a signal having amplitude voltage which is obtained from the secondary winding and which is approximately proportional to amplitude voltage of the resonance inductor (Lh′), is rectified by using a resistor, a diode, a capacitor, etc., thereby forming the voltage detection unit. However, in this example, since the above described high voltage is generated at the resonance inductor at start-up time, in the resonance inductor (Lh′), which has the transformer structure, it is necessary to sufficiently secure insulation of a secondary winding to the high voltage generating section, and to prevent breakdown or corona discharge. Therefore, there is a problem in which a method of sufficiently providing a barrier tape or a tape between winding layers, or a method of separating each winding, section by section, is adopted, thereby causing an increase in cost.
As another example of the detection unit for detecting that the resonant condition is realized, by using a phenomenon in which large current flows from the inverter (Ui′) when the driving frequency of an inverter (Ui′) is in agreement with the resonance frequency of a resonant circuit, providing a current detection unit for the inverter (Ui′) is considered. However, when a resistor having small resistance is used as the current detection unit, there is a problem in which unnecessary resistive loss may be caused since current also flows therethrough steadily in a steady operation during which a discharge lamp is lighted, or cost increases in a case of a system in which a current transformer is arranged at an output of the inverter (Ui′).
As still another example of the detection unit for detecting that the resonant condition is realized, a system is proposed, in which a current phase detection unit for an inverter, and a voltage phase detection unit for the inverter are provided so that a detected inverter current phase and an inverter voltage phase are compared with each other, whereby a feedback operation is performed to actually realize a predetermined phase relation. However, similar to above, in this system, a circuit for the comparison/judgment of a phase, and a current transformer for current detection or a resistor for current detection are required, so that there is a drawback of an increase in cost.
As stated above, various technologies having a detection unit for detecting whether a resonant condition is realized, in which the driving frequency of an inverter is set up so as to be in agreement with resonance frequency in order to continuously generate high voltage, have been conventionally proposed.
For example, Japanese Patent Application Publication No. S52-121975 discloses that where operation frequency is changed and then the operation frequency is fixed when a resonance condition is detected, an inverter is driven at the triple harmonic resonance frequency, and the inverter looks for the resonance frequency so that an operation is performed at the frequency.
For example, Japanese Patent Application Publication No. S55-148393 discloses that in a case where a resonant condition is maintained like self-oscillation, a unit for detecting current which flows through a resonant circuit at time of start-up of a discharge lamp containing gas, is prepared, wherein when a change rate thereof is the maximum or close to the maximum, frequency of an inverter is maintained at the resonance frequency of the resonant circuit by commutating the voltage which is applied to the resonant circuit.
Moreover, Japanese Patent Application Publication No. 2000-012257, similar to the above, in the case where a resonant condition is maintained like self-oscillation, where a discharge lamp is started in a resonant condition, synchronization is automatically performed by self-oscillation of a resonant circuit which is made up of an inductor and a capacitor.
Furthermore, Japanese Patent Application Publication No. 2001-501767 discloses that a detection unit is configured so that a state of a gas discharge lamp is detected, and a control circuit unit controls frequency of an inverter as a function of an output of the detection unit. The Japanese Patent Application Publication also discloses that a feedback circuit unit for effectively changing frequency of an inverter in response to an electric power detection unit is provided, wherein electric power supplied to a gas discharge lamp is maintained to approximately a predetermined level. Further, the Japanese Patent Application Publication No. 2001-501767 discloses that the inverter is configured so as to be continuously operated at frequency which decreases so that the frequency approaches a resonance frequency until a gas discharge lamp starts and thereafter; the inverter is configured so as to be operated at a frequency that decreases so as to approach frequency close to specific frequency until at least the operation of the gas discharge lamp shifts from glow discharge mode to arc discharge mode; and the inverter is operated at a frequency higher than other resonance frequency after the operation of the gas discharge lamp shifts from glow discharge mode to arc discharge mode, so that the gas discharge lamp starts, and shifts from the glow discharge mode to the arc discharge mode, and further is operated in a steady state. Or, the Japanese Patent Application Publication discloses that a step in which the inverter is operated at frequency which decreases so that it may approach from specific frequency to resonance frequency until the gas discharge lamp starts; a step in which the inverter is operated at a frequency that increases so as to approach the specific frequency until the gas discharge lamp shifts from glow discharge to arc discharge; and a step in which the inverter is operated at a frequency higher than other resonance frequency at which the gas discharge lamp is stably operated.
Furthermore, Japanese Patent Application Publication No. 2001-511297 discloses that a system of a detection and determination method of resonance frequency at the driving frequency of a bridge is proposed, wherein a search method is performed based on random sampling and, for example, is continuously carried out until breakdown of the gas electric light lamp and an ignition of the gas discharge lamp occur.
Furthermore, Japanese Patent Application Publication No. 2001-515650 discloses that bridge frequency is decreased in each phase of non-load, glow discharge and arc discharge, wherein first, a resonance igniter is controlled so as to be excited at a frequency sufficiently higher than a nominal resonance frequency, and an excitation frequency is decreased while supervising lamp terminal voltage, or where frequency is decreased toward the nominal resonance frequency and the terminal voltage of the lamp increases, when and the measured lamp terminal voltage reaches a minimum value at the controlled frequency, a controller stops decreasing the frequency and the lamp is continuously excited over designated minimum duration at this frequency.
Furthermore, Japanese Patent Application Publication No. 2004-095334 discloses that a frequency detection unit for detecting frequency of a driving voltage of an inverter and a voltage detection unit for detecting a voltage that is generated by driving a resonant circuit, are provided in a resonant circuit unit, wherein the driving frequency is changed from a high frequency to a low frequency whereby frequency at the time when the voltage detection unit detects the maximum voltage, is set as the driving frequency. The Japanese Patent Application Publication No. 2004-095334 also discloses that the driving frequency is changed from high frequency to low frequency whereby frequency at the time when the voltage detection unit detects a threshold voltage, is set as the driving frequency. Further, the Japanese Patent Application Publication also discloses that in the above-mentioned frequency detection, constant voltage smaller than starting voltage, which may start a discharge lamp, is applied to a resonant circuit, or the secondary winding of a resonance inductor is used as a voltage detection unit, or a measurement is performed at a connection node of a resonant capacitor and a resonance inductor.
Moreover, Japanese Patent Application Publication No. 2004-127656 discloses that after frequency of output voltage of an inverter circuit is set to a frequency lower than the oddth resonance frequency of a resonant circuit in order to turn on a discharge lamp, the frequency of output voltage is increased gradually or stepwise, and the frequency of the output voltage, at a time when the amplitude of oscillating voltage of the resonant circuit becomes a predetermined value or greater, is set as the frequency of the output voltage of the inverter circuit. Also, the Japanese Patent Application Publication No. 2004-127656 discloses that when the amplitude of the output voltage of the resonant circuit does not reach a predetermined value or greater within a predetermined time, after the frequency of the output voltage reaches an upper limit, if the amplitude of the output voltage of the resonant circuit becomes the predetermined value or greater in a process in which the frequency is decreased to targeting an initial frequency, which is a frequency at a time of start-up, at a speed equivalent to the speed at time when the frequency was increased, a frequency which is a few hundredth of percent lower than the frequency at that time, is set. On the other hand, in the process in which the frequency is decreased, when the amplitude of the output voltage of the resonant circuit does not reach the predetermined value or greater, but reaches the initial frequency, an operation in which the frequency is increased again is repeated until the lamp is turned or a predetermined maximum time lapses.
Furthermore, Japanese Patent Application Publication No. 2004-327117 discloses that operation frequency of high-frequency voltage which is generated in an inverter circuit unit is set up to resonance frequency of a resonant circuit or frequency which is approximately odd times the frequency thereof, so that a high voltage pulse can be outputted, and a frequency sweep operation is carried out so that a high voltage pulse can be approximately uniformly outputted, wherein resonance boosting voltage is detected, and when it becomes approximately a target voltage value, the resonance boosting voltage is stopped, or operation frequency is fixed, and an output having approximately a target voltage value continues for a fixed period, or when it becomes approximately the target voltage value, the operation frequency is swept in a direction opposite to the previous sweeping direction so that the output which is approximately the target voltage value or less continues for the a fixed period, or a resonance voltage detection unit is formed by a secondary winding of an inductor of the resonant circuit, or the resonance voltage detection unit is formed by a voltage dividing resistors connected to both ends of a capacitor of the resonant circuit, or a frequency sweep operation is controlled by a microprocessor.
Furthermore, Japanese Patent Application Publication No. 2005-520294 discloses that in order to perform automatic synchronization, as to synchronization based on automatic feedback of a third resonance, for example, an antenna circuit is used as a detecting unit for detecting an output of high voltage generated in a resonant circuit, and a feedback operation is carried out using a PLL circuit.
Japanese Patent Application Publication No. 2005-515589 discloses that, in an automatic synchronization unit, a feedback operation is carried out by using a VCO and a microprocessor, or voltage, current, and high voltage is fed back.
Furthermore, Japanese Patent Application Publication No. 2005-507554 discloses a ballast apparatus, in which the coefficient of self-induction of a coil, and a value of electrostatic capacity of a capacitor, and time jitter switching frequency are related to one another at a certain time during frequency change, so that at least the oddth harmonic frequency of the time jitter switching frequency approaches resonance frequency of the coil and the capacitor.
Furthermore Japanese Patent Application Publication No. 2005-507553 discloses a system in which a unit for measuring voltage of both ends of a discharge lamp is provided in order that a bridge in which an igniter is being operated, performs a high order resonance operation, wherein the driving frequency of the bridge for performing resonance operation is swept before discharge starting, so that frequency is fixed when target voltage is reached, or a method in which after lighting, it is gradually shifted to a low frequency operation.
Furthermore, Japanese Patent Application Publication No. 2007-103290 discloses that a unit for measuring voltage generated in a resonant circuit is provided, so that frequency of a bridge is swept in order to perform a resonance operation at a time of non-load, and the frequency is fixed when the target voltage is reached.
Furthermore, Japanese Patent Application Publication No. 2007-173121 discloses that the driving frequency of an inverter is changed continuously or stepwise from high frequency to low frequency, and based on a value obtained from resonance voltage, it is determined whether the resonance voltage reaches a second voltage level, and after a determination result of reaching the level is obtained, variable frequency is fixed so that the resonance voltage may be maintained to the second voltage level.
Furthermore, Japanese Patent Application Publication No. 2007-179869 discloses that, in a starting sequence of a discharge lamp, a frequency control circuit carries out a sweep operation, by which a frequency control signal is changed, while monitoring a synchronization degree signal, so that frequency is changed, starting from either an upper limit frequency or a lower limit frequency of a frequency variable oscillator, in a range which does not exceed the other frequency, and after completion of the sweep operation, the frequency control circuit determines a value of a frequency control signal with respect to resonance frequency of a resonant circuit, and inputs it into a frequency variable oscillator. In addition, the Japanese Patent Application Publication discloses that, after determining the value of the frequency control signal, the sweep operation covering a narrow range continues, in order to respond to drift of the resonance frequency, and further, the resonant circuit is configured to have the structure using a parallel resonant circuit, and a resonance inductor is configured to have a transformer structure so that the synchronization degree signal may be monitored.
Furthermore, Japanese Patent Application Publication No. 2008-027705 discloses that as a first voltage measurement unit, a resistor and a capacitor are connected to a secondary winding of a resonance inductor, so as to be used for feedback of an output of high voltage due to a resonant action.
Furthermore, Japanese Patent Application Publication No. 2008-269836 discloses that a capacitor and a resistor are connected to a secondary winding of a resonance inductor, so as to be used for feedback of an output of high voltage due to a resonant action, wherein resonance voltage is indirectly detected, and inverter driving frequency is fixed to a frequency at the time when target voltage is met.
Thus, the proposals of the prior art are explained above, that is, a detection unit for detecting whether the resonant condition is realized, is provided, and the driving frequency of an inverter is set up so as to be in agreement with resonance frequency, so that high voltage is continuously generated. However, as described above, the detection unit for detecting whether the resonant condition is realized, is required, so that there is a problem of causing an increase in cost.
These are problems in steps up to generating the high voltage accompanying resonance phenomena in a resonant circuit so as to apply it to a lamp, thereby causing breakdown in the lamp. However, in order to realize a stable lighting state as a discharge lamp lighting apparatus, there remains a problem in that after a breakdown occurs in a lamp so that electric discharge begins, a step in which the driving frequency of an inverter is shifted from a high resonance frequency to a low frequency at which it is in a final stable lighting state, must be certainly and safely completed.
As the technology of switching of the driving frequency of an inverter at a time a high voltage is applied to a discharge lamp and at time of a stable condition, there is a case where a function of resonance starting for certainly shifting from glow discharge to arc discharge is included in a process, or a case where a function where asymmetrical electric discharge phenomenon used in a starting method of applying a high frequency wave, in which current flows only in a direction of one side of a discharge lamp electrode, converges for a short time, and functions of shifting and lighting are stably carried out in both directions of the discharge lamp electrode while suppressing damages to an electrode, are included in the process.
In order to improve them, a method of effectively switching or changing the frequency of an inverter, or a method of switching a current value to a discharge lamp, have been proposed conventionally.
Japanese Patent Application Publication No. H03-167795 discloses that when a start of discharge in a discharge lamp is detected, operation frequency of switching elements is gradually changed from a frequency at time of non-load to a frequency at time of lighting, wherein when asymmetrical electric discharge occurs, passage of extreme overcurrent in a lighting direction is prevented in order not to drop frequency rapidly.
Furthermore, Japanese Patent Application Publication No. H04-121997 discloses that after a lamp is initiated, the frequency is changed to low frequency from resonance frequency or the frequency close thereto, or the frequency is continuously decreased.
Furthermore, Japanese Patent Application Publication No. H04-342990 discloses that at a start-up time of a discharge lamp, an inverter is driven wherein output frequency is close to resonance frequency in an LC series resonant circuit, and if an output of a lamp current detection unit exceeds a predetermined value, an output or frequency of the inverter is switched to a decreased and predetermined value.
Furthermore, Japanese Patent Application Publication No. H07-169583 discloses that a frequency control unit for changing frequency of output voltage of a direct current/alternating current conversion circuit is provided, wherein when a light-out state of a discharge lamp is judged by a lighting judgment unit, a frequency control unit increases frequency of output voltage of the direct current/alternating current conversion unit to a value which is sufficient to cause series resonance by an inductor and a capacitor, and moreover, when the lighting state of a discharge lamp is judged by the lighting judgment unit, the frequency control unit decreases the frequency of the output voltage of the direct current/alternating current conversion circuit.
Furthermore, Japanese Patent Application Serial No. H07-230882 discloses that in a predetermined period after start-up, an inverter unit is continuously operated at frequency that is a resonance frequency or more of a series resonant circuit, and which is close to the resonance frequency.
Furthermore in Japanese Patent Application Publication No. H08-124687 discloses that a resonant circuit makes a full bridge operate at a high order resonance frequency only at a time of non-load, and when a lamp is turned on, a frequency switching control circuit applies voltage of low frequency to the lamp.
Furthermore, Japanese Patent Application Publication No. H11-265796 discloses that when it is judged that a discharge lamp is shifted to a lighting state, it is changed to a predetermined value in which frequency is decreased.
Furthermore, Japanese Patent Application Publication No. 2004-265707 discloses that a full bridge is operated at high order resonance frequency, using an LC resonance circuit, and after lighting, voltage of low frequency is applied to a lamp, wherein a period during which a resonant circuit generates high voltage, and a period during which it outputs direct current voltage or a different period are repeated by turns.
Furthermore, Japanese Patent Application Publication No. 2008-171742 discloses that after a predetermined time lapses from time when a lamp is started, it is determined that the electric discharge occurs at a base portion or at a tip portion, wherein when it is the electric discharge at a tip portion, the operation is changed from a high frequency wave operation to a low frequency steady operation, but when it is the electric discharge at a base portion, the high frequency wave operation continues.
Furthermore, Japanese Patent Application Publication No. 2007-005260 discloses that if a judging circuit for judging that full wave electric discharge or asymmetrical electric discharge occurs in a discharge lamp, if it judges that it is the full wave electric discharge, constant current, which is set up so that the discharge lamp is made to shift to a stable lighting state within a predetermined period, is supplied to the discharge lamp, and on the other hand, if the judging circuit judges it is the half wave discharge, a switching unit switches current which flows between both electrodes so that the current with a peak value larger than the above-mentioned constant current is supplied to the discharge lamp DL.